1. Field of the Invention
The present invention is directed to an iterative shimming method for generating a homogenized basic field in a nuclear magnetic tomography apparatus.
2. Description of the Prior Art
In a nuclear magnetic resonance tomography apparatus, the homogeneity of the basic magnetic field is a critical factor for the imaging quality. Field inhomogeneities in the image region cause geometrical image distortions in the imaging which are proportional to the field deviations. A high field homogeneity is also required in order to separate the resonant lines of fat and water. This enables a suppression of the nuclear magnetic resonance signals which arise from fat tissue, which are generally not of interest. The field inhomogeneity for this purpose must be less than 3.5 ppm over the examination volume. A further complicating factor is that the inhomogeneity arising from the basic field magnet is additively superimposed with inhomogeneities arising from the examination subject. This leads to susceptibility artifacts. In certain instances, an improvement in the basic field inhomogeneity is necessary in vivo, i.e., when the examination subject, generally a human body, is in the basic field.
As described in the article "Aspects of Shimming a Superconductive Whole Body MRI Magnet," Frese et al, Proceedings of the 9th Int. Conf. on Mag. Techn., Zurich, September, 1985, pages 249-251, a magnetic field can be represented using the coefficients arising in the expansion in spherical harmonics of the equation for the magnetic field. This article also described the compensation of field deviations by electrical shim coils. Linear field deviations, i.e., field errors of the first order, can also be compensated by charging the gradient coils with an offset current, i.e., with a constant current that is superimposed on a gradient pulse sequence.
Given more stringent demands on the field homogeneity, not only linear field deviations but also field errors of a higher order must be compensated. Separate shim coils, which are to be charged with a suitable current, must be provided for this purpose in addition to the gradient coils. During imaging, the shimming, i.e., the setting of the currents via the individual shim coils as well as, if necessary, the use of the offset current in gradient coils, is preferably implemented before the examination of each individual patient, with the patient in the examination volume.
The setting of the currents for the shim coils, and the offset currents for the gradient coils, in order to achieve an optimum field homogeneity represents a complex problem which has usually often resolved by trial and error. When this shimming must be undertaken with the patient in the basic field, in order to compensate for field inhomogeneities arising from the examination subject, the dwell time of the patient in the nuclear magnetic resonance tomography apparatus is lengthened. This is disadvantageous both in view of the physical stress on the patient (particularly patients with a tendency to claustrophobia), as well as in view of the patient throughput.
A non-iterative method for general shimming of magnets is described in the article, "Fast, Non-iterative Shimming of Spatially Localized Signals," in the Journal of Magnetic Resonance, pages 323-334 (1992). In this known method, the phase of nuclear spins in the respective directions of a plurality of projections are identified with stimulated echo sequences. A magnetic field course (path) in these projections can be measured due to the phase course, and given representation of the magnetic field in spherical harmonic functions, the coefficients thereof can be identified. Each shim coil is allocated to a spherical harmonic function of the nth degree and of the mth order. The coefficients identified in this manner are then employed as a criterion for the currents to be supplied to the shim coils.
Non-iterative methods, in general, are faster than iterative (interactive) methods, however, it has been shown in practice that due to the complex physical relationships, non-iterative methods do not always lead to satisfactory results.